Crass established a reputation as a Wagnerian early in hisCAREER, appearing at the Bayreuth Festival between 1954 and 1973. He made guest appearances throughout Germany, and was a regular member of the Köln Opera from 1962 until 1964. In addition to his work as a concert and oratoriosinger, he became known for singing works of Wolfgang Amadeus Mozart.
Crass ended hisCAREER abruptly due to hearing trouble in 1981; after that he devoted himself to coaching younger singers. He died in Rüsselsheim, aged 84.